New developments in organic electroluminescent (hereafter referred to as "EL") devices have been vigorously explored since Tang et al. reported an EL device that obtains a high luminance at a low applied voltage. For example, Tang et al. shows a luminance of 1000 cd/m.sup.2 at an applied voltage of 10 V (cf. Appl. Phys. Lett., 51, 913 (1987)). The organic EL device has a light-emitting thin film that features a low drive voltage, high resolution and a wide field of view. These characteristics make it especially usefull in flat-panel displays. However, to use the organic EL device in other applications, it must also have the capability to emit polychromatic light.
A polychromatic display can be constructed with EL elements formed in a planar arrangement. In this configuration, each EL element emits a predetermined pattern of light in one of the three primary colors: red, green or blue. An alternative arrangement combines three color filters that pass red, green or blue light, respectively, from an element that emits white light.
The manufacturing process for the patterned organic EL elements is complicated and makes mass production very difficult. Moreover, the patterning process lowers the light-emitting efficiency of the organic EL elements. The color filter method requires an element that emits a bright and stable white light. No such element has yet been obtained.
A color conversion method has been proposed that includes filters made of fluorescent material. For example, Japanese Unexamined Laid Open Patent Applications No. H03-152897 and No. H05-258860 propose fluorescent material that absorbs light from an organic EL element and emits fluorescent light in the visible spectrum. The light emitted from the organic EL element need not be limited to white for the color conversion method to work. A single color light-emitting element that emits a brighter light may be used as a source. For example, an organic EL element that emits blue light effects the color conversion to a longer wavelength with an efficiency that exceeds 60%.
However, the materials used for converting the emitted light to a desired wavelength are very sensitive to conventional environments. All the conventional materials that have been examined by the present inventors for use as color converters display an undesirable sensitivity to specific light wavelengths, heat, moisture and organic solvents. The sensitivity of these fluorescent materials is such that they are easily decomposed when exposed to environments with the above characteristics. This sensitivity places severe limitations on the design and manufacture of a polychromatic light-emitting device based on the color conversion method.
The Japanese Unexamined Laid Open Patent Application No. H08-279394 proposes a polychromatic light-emitting device that protects the fluorescent materials within the device with an insulative inorganic oxide layer. In this conventional structure, an insulative inorganic oxide layer protects the fluorescent layers from degradation due to exposure. The insulative inorganic oxide layer is interposed between the fluorescent layers and an organic EL element. A fluorescent protection layer and an adhesive layer are applied between the fluorescent layers and the insulative inorganic oxide layer. The above patent application discloses the following four laminate structures:
(1) A transparent substrate/fluorescent layers/a transparent and electrically insulative inorganic oxide layer/an organic EL element; PA1 (2) A transparent substrate/fluorescent layers/an adhesive layer/a transparent and electrically insulative inorganic oxide layer/an organic EL element; PA1 (3) A transparent substrate/fluorescent layers/a protection layer (transparent flattening layer)/an adhesive layer/a transparent and electrically insulative inorganic oxide layer/an organic EL element; and PA1 (4) A transparent substrate/fluorescent layers/a protection layer (transparent flattening layer)/a transparent and electrically insulative inorganic oxide layer/an organic EL element.
The fluorescent materials that produce the different colors each have different conversion efficiencies. The brightness of the colors produced by the fluorescent layers of a color filter (hereinafter referred to as a "color conversion filter") are balanced by varying the thickness of the specific material needed for a given color. As shown in FIG. 5, the thickness differences cause steps in the surface of the color conversion filter mounted on a transparent substrate 7 such as a glass substrate. Results of experiments conducted by the present inventors show that the wavelength conversion using the red fluorescent material is much less efficient than that of other color fluorescent materials. A red fluorescent layer 4 must therefore be several tens of micrometers thick to obtain emitted light that has a high color purity. However, the thickness of the higher efficiency green fluorescent layer 5 is only 4 to 10 .mu.m. Similarly, a blue fluorescent layer 6 has a thickness of less than 5 .mu.m. The difference in thickness of the fluorescent materials forms a step of 10 .mu.m or more between the red fluorescent layer 4 and the green and blue fluorescent layers 5 and 6, respectively. In the instance where the EL element light source emits blue light, there is no need for a blue fluorescent layer. That is, the thickness of the blue fluorescent layer 6 is 0 .mu.m.
When a transparent insulative inorganic oxide layer 14 is formed on the fluorescent layers above, the surface inevitably contains concave and convex portions, as shown in FIG. 6. The concave and convex portions of the surface adversely affect the color purity and the angle of visibility of the light-emitting device. It is difficult to form the transparent insulative inorganic oxide layer 14 with a flat surface. The adverse effects on color purity and angle of visibility are therefore difficult to remove.
Protection and adhesive layers are formed in turn over the fluorescent material layers of the color conversion filter. During this process, the fluorescent materials are repeatedly exposed to heat and light. The exposure of the fluorescent materials to heat and light is necessary to form and cure the protection and adhesive layers. However, it is difficult to completely prevent deterioration in the fluorescent materials caused by the repeated exposure to heat and light.
A protective resin coat can be applied over a color conversion filter on a transparent substrate. The protective resin coat is thinner where it covers the stepped fluorescent layers of the filter, and can not adequately accomplish its protective function. If the protective layer is to function properly, it must be thickened to avoid the formation of thin portions. However, increasing the thickness of the protection layer also increases the distance between the color conversion filter and the EL elements. As the distance between the color conversion filter and the EL element grows, the angle of visibility narrows. Moreover, the materials and construction method for the protection layer are limited due to the high sensitivity of the fluorescent materials to chemical reagents, moisture, light and heat. The complicated manufacturing steps necessary to form the above described layer structures make the conventional device structures industrially disadvantageous.